1. Field of the Invention
This invention relates to an A/D converter system and more particularly to a system which has error correction and calibration apparatus incorporated therein and associated therewith and to methods of operation thereof. The converter system produces highly accurate and reliable results, operating at high speeds and with a high dynamic range, while the size, complexity and cost of manufacture of the system are minimized.
2. Description of the Prior Art
A number of improvements have been made in architectures of analog to digital converters to increase the speed and resolution of the conversions obtainable therefrom. One highly advantageous type converter has a subranging architecture in which digital signals obtained from main range and subrange A/D converters are logically combined to produce a digital output. The main range A/D converter, which is desirably a "flash" type of converter, operates to produce a digital signal which is a first approximation of the magnitude of an input analog signal. A subrange D/A converter, preferably a high speed precision converter, converts the digital signal so produced to an analog signal which is compared to the input analog signal to produce a difference signal which is applied to the subrange A/D.
With "half-flash" subranging architecture in which the main A/D converter is of the flash type, higher resolution conversion became possible but with limitations as to the obtainable speed and accuracy. Further improvements are obtainable with a microprocessor controlled correction system for reducing errors as disclosed in the Evans U.S. Pat. No. 4,612,533. In this system, a correction D/A converter supplies an analog correction signal to be summed with an analog signal from a subrange D/A and an input analog signal. The correction D/A converter is supplied with a digital signal from the data output of a RAM which is supplied with a control address signal derived through a digital multiplexer either from the digital output of a main range A/D converter or from the microprocessor. An analog multiplexer is controlled by the microprocessor to connect the input of an input sample and hold circuit either to an analog input or to the output of a low distortion sine wave oscillator. The microprocessor is operative to connect the input of an input sample and hold circuit through an analog multiplexer to a distortion sine wave oscillator and to then perform a FFT (Fast Fourier Transform) and to produce correction values which are stored in the RAM. Then the input of the sample and hold circuit is connected to an analog input while the stored correction values are accessed to apply digital signals to the correction D/A converter and to apply the analog correction signal.
With the microprocessor controlled error correction system, improved accuracy is obtainable and the subrange D/A converter and also the correction D/A converter need not be of expensive high precision types. However, the addition of the microprocessor and associated circuitry substantially increases the cost of manufacture of the system.